Conventional petroleum hydrocracking and catalytic dewaxing are described by fundamentally different process chemistry. Due to their molecular structure, naphthenes and aromatics are easier to hydrocrack than paraffins. Less energy is required to open rings and saturate aromatics than is required to break paraffinic bonds. As a result, in conventional high conversion recycle hydrocracking, the recycle oil stream becomes rich in paraffins and is characterized by a very high pour point, which at time may exceed 38.degree. C. (100.degree. F.). "Shape selective" catalysts that restrict the cracking of ring structures and preferentially convert normal paraffins, resulting in substantial pour point and molecular weight reduction may be employed in catalytic dewaxing, particularly in the manufacture of lubricants. Specific types of dewaxing catalysts are designed to convert the normal paraffins by isomerization in conjunction with cracking and as a consequence are selective to middle distillate products such as kerosene and diesel oil.
The instant invention discloses an integrated hydroprocessing scheme which employs both hydrocracking and catalytic dewaxing. Unconverted hydrocracker bottoms are dewaxed in a dewaxing stage. Unconverted material from the dewaxing stage is subsequently recycled back to the hydrocracking stage. The feed to the catalytic dewaxing stage, the hydrocracked bottoms fraction, is generally rich in normal paraffins. These normal paraffins may be isomerized in the dewaxing stage to produce more branched chain paraffins which are suitable gasoline and middle distillate components.
The dewaxing stage may employ a single catalyst or a two catalyst system. A large pore catalyst such as zeolite B may be employed to isomerize n-paraffins or crack them to some extent depending on the catalyst activity. Shape selective, intermediate pore catalysts, such as ZSM-5, may also be used to crack n-paraffins.
Integrated hydroprocessing schemes which employ both hydrocracking and catalytic dewaxing steps are well known in the refining arts. U.S. Pat. No. 5,275,719 (Baker et al, hereinafter Baker) discloses a process for producing a high viscosity index lubricant which possesses a V.I. of at least 140 from a hydrocarbon feed of mineral oil origin which contains nitrogen compounds and has a wax content of at least 50 wt %. The feed is hydrocracked in an initial stage at a hydrogen partial pressure of at least 800 psig over a bifunctional lube hydrocracking catalyst comprising a metal hydrogenation component on an acidic, amorphous porous support material. Not more than 50 wt % of the feed is converted to products boiling outside the lube boiling range. The effluent of the first stage, which contains nitrogen compounds, is treated with a low acidity, isomerization catalyst having an alpha value of not more than 20, the catalyst comprising zeolite beta and a noble metal.
Waxy paraffins in the first stage effluent are thus isomerized to less waxy paraffins. Aromatics content is reduced to less than 1 wt %. Isomerization of waxy paraffins and hydrotreating occur simultaneously in the second stage of Baker. The temperature range required for simultaneous isomerization and hydrotreating is maintained by stripping gas and liquid compounds which contain nitrogen from the first stage effluent and returning such compounds as necessary to the second stage.
The overwhelming motivation of Baker is the maintenance of the lubricant VI by control of the conversion of the feed to lower boiling materials. Distillate production is only incidental in Baker. The process of the patent is actually directed to lubricant production. This is evident from the type of feedstocks used. The feedstocks employed in the instant invention typically contain no more than 30 wt % paraffins, whereas the feeds of Baker possess a wax content of at least 50 wt %. Presumably these waxes are paraffins. A preferred feed in Baker is slack wax, which typically possesses a nitrogen content of less than 30 ppmw, as illustrated by Tables 2 and 3 of Baker. The preferred feed in the instant invention is vacuum gas oil or light cycle oil. These feeds typically possess nitrogen contents greater than 300 ppmw and sometimes greater than 1000 ppmw, as the Tables 1-7 illustrate. Much of this nitrogen is found in the back ends of the aromatics, however.
Baker discloses only the use of amorphous hydrocracking catalysts in its process. The feeds of Baker, such as slack wax, contain nitrogen molecules which are easily removed by hydrocracking with an amorphous catalyst. Hydrocracking catalysts which comprise zeolites are not contemplated and would not be impractical for use in the Baker process, however. Hydrocracking catalyst comprising zeolites preferably ultrastable, large pore zeolites (such as REX, REY and USY are preferred in the instant invention because of their high cracking activity. Conversion to lower boiling products in the instant invention may be as great as 80 wt %, while in Baker, conversion is to be no more than 50 wt %. The feeds of the instant invention, such as vacuum gas oil, contain far greater amounts of total nitrogen than do the feeds of Baker. The nitrogen of VGO feeds, is however, more tightly bound because it is generally located in the heavy back ends of aromatics. It is less likely to contaminate the internal pores of a zeolite than are the loosely bound nitrogen molecules of a feed such as slack wax. As the pores of a zeolite become contaminated, conversion rates will decrease. More zeolite must be added to maintain the conversion level. A preliminary layer of hydrotreating catalyst might be necessary to treat the feed before it contacts the catalyst comprising the zeolite.
U.S. Pat. Nos. 4,764,266 (Chen et al. I) and 4,851,109 (Chen et al. II) disclose integrated hydroprocessing scheme in which high boiling feeds such as gas oil and catalytically cracked cycle oils are hydrocracked over aromatic selective hydrocracking catalysts, producing naphtha and middle distillate range products. In Chen et al. I, the preferred aromatic selective hydrocracking catalyst comprises USY. Unconverted material may be recycled to the hydrocracking stage or may be passed to the dewaxing stage, where it is treated with a large pore catalyst, specifically zeolite beta, which has a hydrogenation-dehydrogenation functionality, in order to produce more kerosene and distillate range products. Although recycle of hydrocracker bottoms is employed in these patents, unconverted material from the dewaxing stage is not recycled to the hydrocracker. Furthermore, these processes do not illustrate the use of an atmospheric distillation tower having a baffle in the flash zone in order to separate the bottoms material from two different process streams, as illustrated in one embodiment of the instant invention.
PCT application WO 95/10578 discloses a process for the production of middle distillate products by the dewaxing of hydrocarbonaceous material boiling above 343.degree. C. The feed is hydrocracked with a catalyst comprising a large pore zeolite such as Y and a hydrogenation metal component selected from Group VIB or Group VIII. The entire effluent of the hydrocracking zone is then dewaxed in the presence of hydrogen with a catalyst comprising an intermediate pore zeolite of the ZSM-5 type. In the instant invention, only the bottoms of the hydrocracker effluent is dewaxed.
European Patent 189648 A1 discloses a process for the hydroprocessing of heavy gas oil feeds in order to maximize kerosene and distillate production. FIG. 5 of this patent illustrates hydrocracking of the feed, followed by catalytic dewaxing. All of the effluent of the hydrocracking unit is dewaxed in this invention, not just the bottoms, as in the instant application. Furthermore, there is no teaching of recycling to the initial hydrocracking stage. Two different hydrocracking stages are used.
U.S. Pat. No. 5,053,117 (Kyan et al) discloses a process for dewaxing hydrocracker bottoms from the MPHC(Moderate Pressure Hydrocracker Bottoms) process to produce either naphtha and gas oil under MDDW conditions or lubricants under the less severe conditions of MLDW. The feed to the dewaxing unit may be combined with gas oil of high nitrogen content, thereby improving the octane number of the naphtha product. There is no teaching of the recycling of dewaxer bottoms to the hydrocracker. Gasoline octane number may be improved by maximizing olefin production. This may be accomplished by employing MDDW catalyst and conditions with minimal hydrogenation.
A variety of patents illustrate a generalized process scheme in which a feed such as a heavy gas oil (or other hydrocarbon feed boiling above 343.degree. C. (650.degree. F.)) is hydrocracked, then dewaxed to produce lubricating oils. U.S. Pat. No. 4,414,097 (Chester et al) discloses a process in which a hydrocarbon feedstock which boils above 343.degree. C. (650.degree. F.) is hydrocracked, then dewaxed over a catalyst comprising ZSM-23.
U.S. Pat. Nos. 4,282,271 (Garwood et al) and 4,283,272 (Garwood et al) disclose hydroprocessing of lubricating oils by hydrocracking of feedstocks boiling above 343.degree. C. (650.degree. F.). The hydrocrackate is subsequently dewaxed. The effluent from the dewaxer is hydrotreated but is not recycled to the hydrocracking stage.